In-plane MEMS thermal actuator and associated fabrication methods

ABSTRACT

A MEMS thermal actuator device is provided that is capable of providing linear displacement in a plane generally parallel to the surface of a substrate. Additionally, the MEMS thermal actuator may provide for a self-contained heating mechanism that allows for the thermal actuator to be actuated using lower power consumption and lower operating temperatures. The MEMS thermal actuator includes a microelectronic substrate having a first surface and at least one anchor structure affixed to the first surface. A composite beam extends from the anchor(s) and overlies the first surface of the substrate. The composite beam is adapted for thermal actuation, such that it will controllably deflect along a predetermined path that extends substantially parallel to the first surface of the microelectronic substrate.

FIELD OF THE INVENTION

The present invention relates to microelectromechanical actuators, andmore particular to a thermal actuator having self-contained heatingcapabilities and providing in-plane actuation.

BACKGROUND OF THE INVENTION

Microelectromechanical structures (MEMS) and other microengineereddevices are presently being developed for a wide variety of applicationsin view of the size, cost and reliability advantages provided by thesedevices. Many different varieties of MEMS devices have been created,including microgears, micromotors, and other micromachined devices thatare capable of motion or applying force. These MEMS devices can beemployed in a variety of applications including hydraulic applicationsin which MEMS pumps or valves are utilized and optical applications thatinclude MEMS light valves and shutters.

MEMS devices have relied upon various techniques to provide the forcenecessary to cause the desired motion within these microstructures. Forexample, cantilevers have been employed to transmit mechanical force inorder to rotate micromachined springs and gears. In addition, somemicromotors are driven by electromagnetic fields, while othermicromachined structures are activated by piezoelectric or electrostaticforces. Recently, MEMS devices that are actuated by the controlledthermal expansion of an actuator or other MEMS component have beendeveloped. For example, U.S. patent application Ser. Nos. 08/767,192;08/936,598, and 08/965,277 are assigned to MCNC, the assignee of thepresent invention, and describe various types of thermally actuated MEMSdevices. In addition, MEMS devices have been recently developed thatinclude rotational connections to allow rotation with less torsionalstress and lower applied force than found with torsion bar connections.For instance, U.S. patent application Ser. No. 08/719,711, also assignedto MCNC, describes various rotational MEMS connections. The contents ofeach of these applications are hereby incorporated by reference herein.

Thermally actuated MEMS devices that rely on thermal expansion of theactuator have recently been developed to provide for actuation in-plane,i.e. displacement along a plane generally parallel to the surface of themicroelectronic substrate. However, these thermal actuators rely onexternal heating means to provide the thermal energy necessary to causeexpansion in the materials comprising the actuator and the resultingactuation. These external heaters generally require larger amounts ofvoltage and higher operating temperatures to affect actuation. Forexamples of such thermally actuated MEMS devices see U.S. Pat. No.5,881,198 entitled “Microactuator for Precisely Positioning an OpticalFiber and an Associated Method” issued Mar. 9, 1999, in the name ofinventor Haake, and U.S. Pat. No. 5,602,955 entitled “Microactuator forPrecisely Aligning an Optical Fiber and an Associated FabricationMethod” issued Feb. 11, 1997, in the name of inventor Haake.

As such, a need exists to provide MEMS thermal actuated devices that arecapable of generating relatively large displacement, while operating atsignificantly lower temperatures (i.e. lower power consumption) andconsuming less area on the surface of a microelectronic substrate. Theseattributes are especially desirable in a MEMS thermal actuated devicethat provides relatively in-plane, linear motion and affords the benefitof having a self-contained heating mechanism.

SUMMARY OF THE INVENTION

A MEMS thermal actuator device is therefore provided that is capable ofproviding linear displacement in a plane generally parallel to thesurface of a substrate. Additionally, the MEMS thermal actuator of thepresent invention may provide for a self-contained heating mechanismthat allows for the thermal actuator to be actuated using lower powerconsumption and lower operating temperatures.

The MEMS thermal actuator includes a microelectronic substrate having afirst surface and an anchor structure affixed to the first surface. Acomposite beam extends from the anchor and overlies the first surface ofthe substrate. The composite beam is adapted for thermal actuation, suchthat it will controllably deflect along a predetermined path thatextends substantially parallel to the first surface of themicroelectronic substrate. In one embodiment the composite beamcomprises two or more layers having materials that have correspondinglydifferent thermal coefficients of expansion. As such, the layers willrespond differently when thermal energy is supplied to the composite. Anelectrically conductive path may extend throughout the composite beam toeffectuate thermal actuation.

In one embodiment of the invention a two layer composite beam comprisesa first layer of a semiconductor material and a second layer of ametallic material. The semiconductor material may be selectively dopedduring fabrication so as to create a self-contained heating mechanismwithin the composite beam. The doped semiconductor region may afford apath of least resistance for electrical current. The doping process mayalso enhance the fabrication of contacts on the surface of the anchor.Additionally, the composite beam, which is characterized by a highaspect ratio in the z plane direction, may be constructed so that thefirst and second layers lie in planes that are generally perpendicularto the first surface of the microelectronic substrate. The verticallayer of the composite beam provides for deflection of the beam along apredetermined path that extends generally parallel to the surface of themicroelectronic substrate.

In another embodiment of the invention, a MEMS thermal actuator includestwo or more composite beams. The two or more composite beams may bedisposed in an array or a ganged fashion, such that the multiplecomposite beams benefit from overall force multiplication. In one suchembodiment, two composite beams are disposed on the surface of amicroelectronic substrate such that the ends of the beam farthest fromthe anchor structure face one another. An interconnecting element isoperably connected to the free ends of the composite beam. Theinterconnecting element is configured so as to impart flexibility whenthe two composite beams are actuated in unison. The flexible nature ofthe interconnecting element allows for the overall MEMS thermal actuatordevice to impart a greater distance of linear deflection.

In yet another multi composite beam embodiment, two composite beams aredisposed on the surface of a microelectronic substrate such that theends of the beam farthest from the anchor structure face one another andthe beams are proximate a flexible beam structure. The flexible beamstructure comprises a platform disposed between two or more anchorsaffixed to the substrate. One or more flexible beams operably connectthe platform and the anchors. The platform is adjacent to the free endsof the composite beams such that thermal actuation of the composite beamcauses the beams to operably contact the platform and deflect it in agenerally linear fashion. The flexible beam structure that houses theplatform compensates for variances that may occur in the thermalactuation of the composite beams.

The invention is also embodied in a method for fabricating the MEMSthermal actuators of the present invention. The method comprisesaffixing a first microelectronic substrate to a second microelectronicsubstrate. After the second substrate has been thinned to apredetermined thickness, a first portion of the MEMS thermal actuatorconstruct is then defined in the second microelectronic substrate,including the first layer of a composite beam and a portion of theanchor structure. A doping process may be undertaken to define a path ofleast resistance in the first layer of the composite beam. A secondlayer is disposed on the first layer, the second layer and first layercomprising different materials that respond differently to thermalactuation. The variance in thermal coefficients of expansion causing thefirst and second layers of the composite beam to actuate the compositebeam along a predetermined path that extends substantially parallel tothe surface of the microelectronic substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a self-contained, in-plane, MEMS thermalactuator in accordance with an embodiment of the present invention.

FIG. 1A is a perspective view of a self-contained, in-plane, MEMSthermal actuator in accordance with an embodiment of the presentinvention.

FIG. 2 is a cross-sectional view of the self-contained, in-plane, MEMSthermal actuator shown in FIG. 1, in accordance with an embodiment ofthe present invention.

FIG. 3 is a plan view of a dual self-contained, in-plane MEMS thermalactuator including an interconnecting element used to facilitate greaterlinear displacement, in accordance with another embodiment of thepresent invention.

FIG. 4 is a plan view of a dual self-contained, in plane MEMS thermalactuator including a flexible beam construct used to facilitate greaterlinear displacement, in accordance with another embodiment of thepresent invention.

FIGS. 5A-5G illustrate various stages in the fabrication process of theself-contained, in-plane MEMS thermal actuator, in accordance with amethod for making embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein; rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. Likenumbers refer to like elements throughout.

The following description details MEMS thermal actuated devices that arecapable of generating in-plane, linear motion and provide for aself-contained heating mechanism to effectuate thermal expansion of thematerials comprising the actuator. The resulting MEMS thermal actuateddevices are thereby capable of operating at significantly lowertemperature and power while providing relatively large displacements. Inaddition, methods for fabricating such devices are provided herein.

Referring to FIGS. 1, 1A and 2, shown, respectively, are a top planview, a perspective view and a cross-sectional end view of aself-contained, in-plane, MEMS thermal actuator 10 in accordance withone embodiment of the present invention. The thermal actuator comprisesa composite beam 12 that is affixed to the microelectronic substrate 14.The composite beam is affixed, at proximal end 16, to the substrate viaan anchor, shown in FIG. 1 as first anchor portion 18 and second anchorportion 20. From the proximal end, the composite beam extends outward,overlying the microelectronic substrate, and concluding in a distal end22 disposed furthest from the anchor(s). As such, the composite beamoverlies and is suspended above the microelectronic substrate in acantilever-like configuration. Optionally, the thermal actuator of thepresent invention may define a trench 24 disposed in the surface 26 ofthe microelectronic substrate 14 that provides for additional thermalisolation between the composite beam and the microelectronic substrate.For example, the cross-sectional view of FIG. 2 depicts the suspendedcomposite beam disposed above a trench defined in the microelectronicsubstrate.

The composite beam will comprise at least two materials thatcharacteristically have different thermal coefficients of expansion. Asdepicted in FIG. 1, the composite beam includes a first layer 28 and asecond layer 30. It is also possible and within the inventive conceptsherein disclosed to construct the composite beam with more than twolayers. As shown in FIG. 2, the first and second layers are disposedvertically in relationship to the surface of the microelectronicsubstrate. The vertical relationship of the layers is required to affectactuation in an in-plane direction, parallel with the generally planarsurface of the microelectronic substrate, as shown by arrow 31. Thefirst and second layers will, typically, be thin layers of about 1micron to about 10 microns so as to facilitate flexibility and movementin the overall composite beam. Since the layers have different thermalcoefficients of expansion, the layers will respond differently tothermal actuation resulting in deflection of the composite beam.

In one embodiment of the invention the first layer 28 may comprise asemiconductor material, such as silicon, and the second layer 30 maycomprise a metallic material, such as gold or nickel. In this embodimentthe second layer has a characteristically higher coefficient of thermalexpansion than the first layer. Since the layer having the highercoefficient of expansion will expand more readily as temperatureincreases, the distal end of the composite beam will bend toward thelayer having the lower coefficient of thermal expansion when thermalenergy is supplied to the composite beam. In the embodiment described,in which the second layer 30 has the higher coefficient of thermalexpansion, the layering relationship will effect movement of the beam tothe right, toward the first layer 28, when reviewed in FIG. 2. It willbe readily apparent to those having skill in the art, that the layeringmay be reversed, so that the material having the higher coefficient ofthermal expansion is on the opposite side in the depicted embodimentand, thus, the movement of the beam will be to the left as viewed inFIG. 2. Altering various composite beam and thermal actuatorcharacteristics can vary the amount of deflection observed in thecomposite beam. Included among these characteristics are the choice ofmaterials used for the layers, the thickness of the layers, and theamount of heat supplied to the composite beam, and structuraldiscontinuities in the layers.

The first and second anchor portions 18 and 20 serve to affix thecomposite beam 12 to the underlying substrate. The shape of the overallanchor and/or the anchor portions is shown by way of example only. Thegeneral shape and location of the anchors at the proximal end 16 will bedictated by many design factors, including but not limited to, theamount of composite beam deflection desired, the coefficients of thermalexpansion of the composite beam materials, the desired rigidity of theoverall thermal actuator, etc. First and second contacts 32 and 34 aredisposed on respective first and second anchor portions 18 and 20. Thecontacts are interconnected with a source of electrical energy (notshown in FIG. 1) and serve as connection points for an electricalcurrent that runs through the composite beam. The heat generated by theelectrical current in the beam acts as the means for actuating thecomposite beam. In the embodiment shown the two anchor portions areseparated by a gap 36. The gap serves as an insulator providingelectrical insulation between the first and second contacts.

As mentioned above, the microelectronic substrate 14 may define a trench24 in the region underlying the composite beam. Typically, the trenchwill be slightly longer than the length of the composite beam and themaximum deflection distance of the beam will dictate the width of thetrench. The trench provides heating efficiency advantages. For example,the gap created by the trench between the composite beam and themicroelectronic substrate provides thermal isolation and, thus, lessheat loss is experienced between the composite beam and the substrate.Additionally, the trench simplifies the fabrication process used todeposit the metallic second layer 30 upon the first layer 28. Incontrast with conventional horizontal type layering constructs, thelayers of the composite beam are stacked vertically. To effectuatevertical layering the metallic second layer 30 is deposited at an angleto assure uniform coverage of the vertical sidewall 38 of the firstlayer (shown in FIG. 2). Without a trench in place, there is alikelihood that depositing the metallic second layer will lead toelectrical shorting of the underlying microelectronic substrate and anymetal elements defined on the substrate. Alternately, the MEMS thermalactuator of the present invention may be constructed without a trenchdefined in the microelectronic substrate. In such embodiments a releaselayer, typically an oxide layer, is used to release the composite beamfrom the surface of the substrate during fabrication. The releasingprocess results in a small gap, typically less that 1 micron, existingbetween the microelectronic substrate and the composite beam.

The composite beam is adapted for thermal actuation through direct,self-heating techniques using electric current. As previously discussedprior art in-plane, thermal actuators have used indirect heatingtechniques and have, thus, required large currents and high operatingtemperatures. As such, the high temperature, large current nature ofprior art thermal actuators makes them less efficient than the thermalactuator of the present invention. In order to permit direct heating,each layer of the composite beam defines an electrically conductive paththat runs in a continuous loop through the composite beam and between afirst and second contacts 32 and 34 disposed upon the anchor portions 18and 20. In this fashion, an electrical circuit is formed by passingcurrent from one contact and through one of the layers of the compositebeam to the distal end of the composite beam prior to returning to theother contact via the other layer. Preferably, the electricallyconductive path is disposed so as to substantially surround at least oneof the layers comprising the composite beam. It is possible and withinthe inventive concepts herein disclosed to alter the configuration ofthe electrically conductive path to form a circuit loop that willeffectuate heat in the composite beam. The electrically conductive pathhas a predetermined electrical resistance so as to permit thermalactuation of the composite beam when electrical energy is suppliedthereto.

The present invention uses controlled doping to facilitate self-heatingand to customize the resistance characteristics of the non-metallicmaterials used in the thermal acuator construct. In this fashion thedoped region of the composite beam acts as the heater, self-containedwithin the composite beam structure. Materials such as silicon arehighly resistant and, thus, the doping of such materials aids inconstructing a highly conductive path for the passage of electricalcurrent. Highly doping silicon can be achieved with materials such asphosphorus or boron. The use of doping techniques is well known by thoseof ordinary skill in the art. It should also be noted that theembodiments herein described are not limited to internal heating toeffectuate actuation. The MEMS thermal actuators shown here will operatewith external heating and in certain embodiments, depending on thecomposition of the substrate and the materials used to form theactuator, ambient temperature actuation is possible.

In the embodiment shown in FIGS. 1 and 2, in which the first layer 28comprises a semiconductor material, such as silicon, and the secondlayer 30 comprises a metal, such as gold, the external surface 40 of thefirst layer has been controllably doped. The second layer iselectrically connected at junction 41 to the doped conductive surface ofthe first layer at the tip 42 of the distal end of the composite beam.The tip portion of the distal end may be fabricated so that it is eitheran extension of the second layer (as shown in FIG. 1) or it may comprisea continuation of the doped conductive region of the first layer leadinginto the second layer. Both alternative configurations will allow forthe second layer to be electrically connected with the doped conductivesurface of the first layer. Since the doped regions define a path ofleast electrical resistance, the electrical current will mostly followthe path defined by the doped portions of the first layer and themetallic second layer, with the undoped portion 28 being an electricalinsulator. Accordingly, the conductive path is provided between thecontacts 32 and 34 by the doped portion of the first layer and themetallic second layer. As shown, the contacts have also been doped toincrease electrical conductivity. It is also possible to devise contactsthat would not require doping. By way of example, when a source ofelectrical energy is applied between the contacts electrical currentwould flow from the first contact 32 along the external surface 40 ofthe first layer (i.e. the doped region of the first layer), throughjunction 41 into the interconnected second layer 30 and return backthrough the second layer to the second contact 34. In an alternateembodiment the electrical current could flow in the opposite direction,emanating from the second contact, flowing through the composite beamand completing the path at the first contact.

Referring to FIG. 3, in another embodiment of the present invention thethermal actuator includes dual composite beam actuators 102 and 104. Afirst and second composite beam 106 and 108 are disposed proximatelysuch that the distal ends 110 of the respective beams face each other.The composite beams are adapted to move in unison in response to theselective application of thermal actuation. To assure uniform movementbetween the composite beams the beams will generally be comprised ofidentical layers; similar in material, quantity of layers, layerthickness and doping characteristics. The dual composite beams maygenerally be perceived as mirror images of one another.

As shown in FIG. 3, first and second composite beams are adapted todeflect in the direction of the arrow 112 when thermal energy is appliedto the beams. When a single composite beam thermal actuator is usedinitial deflection of the beam is generally linear, however as the beamcontinues to deflect the pattern of deflection takes on an arcuate path.In this regard, as thermal energy is applied to the single compositebeam actuator the beam has a limited range of relatively lineardisplacement before the displacement becomes increasingly more rotary orangular. In the FIG. 3 embodiment affixing an interconnecting element114 to the distal ends of each composite beam increases lineardisplacement. The interconnecting element may be formed during thefabrication process that defines one of the layers of the compositebeam. Thus, the interconnecting element may comprise silicon, gold,nickel or a similar suitable material. In a silicon embodiment theinterconnecting element may be doped or undoped. If the interconnectingelement is doped, the doping will typically occur simultaneous with thedoping of the contacts 32 and 34 and the external surface 40 of thefirst layer. Additionally, if the interconnecting element is doped anactuating electrical conductive path may exist between the contact 32 ofthe first thermal actuator 102, the interconnecting element and thecontact 32 of the second thermal actuator 104. This electricalconfiguration would make the second pair of contacts 34 optional.Alternately, the electrical path may exist between the second pair ofcontacts 34 and the doped interconnecting element thus, eliminating theneed for the first set of contacts 32. Preferably, the interconnectingelement is shaped and sized so as to impart flexibility. The flexiblenature of the interconnecting element increases the linear displacementdistance. In the FIG. 3 illustration, the interconnecting element has apreferred wishbone-like configuration. As the composite beams begin todeflect upward, the legs 116 of the wishbone-like configuration flexoutward and result in an overall linear displacement of theinterconnecting element in the direction of the arrow.

Additionally, the interconnecting element 114 may serve to simplify theoverall heating arrangement of the dual composite beam thermal actuator.The interconnecting element may serve as a bridge that allows theelectrical current to flow from one composite beam to the next. In theembodiment shown in FIG. 3 the interconnecting element uses the dopedregion of the first layer, typically silicon, as the transfer pathbetween the first composite beam and the second composite beam. In sucha configuration the electrical bridging capabilities of theinterconnecting element may eliminate the need to supply electricalcurrent individually to both composite beams. In an embodiment in whichthe interconnect is used as an electrical link it is more desirable tohave the interconnect element comprise a metal or doped silicon so thatelectrical resistance can be minimized.

Another multi beam thermal actuator in accordance with yet anotherembodiment of the present invention is shown in FIG. 4. This embodimentprovides for a flexible beam 130 construct disposed proximate the distalends 110 of the dual composite beams 106 and 108. In much the samefashion as the dual composite beam thermal actuator shown in FIG. 3, thecomposite beams shown in the FIG. 4 embodiment will generally beperceived as mirror images of one another. The dual composite beams arein operable contact with a platform 132. The platform is disposed sothat it is generally midway between the distal end of the firstcomposite beam and the distal end of the second composite beam. Theplatform is operably connected to at least two anchors that are affixedto the microelectronic substrate, shown here as first and second anchors134 and 136. In the embodiment shown in FIG. 4 the anchors are operablyconnected to the platform via first flexible beam 138 and secondflexible beam 140. A series of springs 142 are located along the beamsand serve to provide elasticity to the overall flexible beam structure.The fabrication of the platform, the anchors, the flexible beams and thesprings can be part of the same patterning and etch processes used toform the first layer of the thermal actuator construct or they mayentail separate processing steps. As such, the platform, the anchors,the flexible beams and the springs may comprise silicon, gold, nickel orany other suitable material.

In operation the dual thermal actuators are activated by thermal energyand provide for the generally simultaneous deflection of the distal endsof the composite beams. Upon actuation the distal ends of the compositebeams contact the platform and provide the force necessary to move theplatform in a linear direction (the path of the platform is shown asarrow 144). The beams and the springs allow for the platform to move inthe linear direction and allow for the platform to relax into anon-actuated position upon deactivation of the thermal actuators. Thisembodiment of a dual beam actuator is advantageous because thespring-like beam structure compensates for any unequal movement of therespective composite beams and accordingly provides enhanced lineardisplacement characteristics as compared to the aforementioned describedsingle composite beam embodiment.

Numerous other multi beam thermal actuator embodiments are also feasibleand within the inventive concepts herein disclosed. For example, thecomposite beams may be arranged radially, with the distal ends directedtoward a control hub to effectuate rotational movement. The hub may havelevers extending from it that provide added actuation force. For adetailed discussion of rotary type MEMS structures see U.S. patentapplication Ser. No. 09/275,058 entitled “Microelectromechanical RotaryStructures” filed on May 23, 1999, in the name of inventors Hill et al.and assigned to MCNC, the assignee of the present invention. Thatapplication is herein incorporated by reference as if set forth fullyherein. Additionally, the composite beams may be ganged together and/orconfigured in an array to benefit from force multiplication, thereby,effectively increasing the ability to move objects a greater distanceand/or move larger objects.

FIGS. 5A-5G illustrate cross-sectional views of various fabricationstages in accordance with a method of making the thermal actuator of thepresent invention. Referring to FIG. 5A, a first microelectronicsubstrate 200 having a first oxide layer 202 is formed on the substrateand a trench 204 is defined through the first oxide layer and into thefirst microelectronic substrate. The first microelectronic substrate maycomprise silicon although other suitable substrate material may also beused. The first oxide layer is typically disposed on the substrate by aconventional thermal oxidation process in which the substrate is exposedto an oxygen environment at elevated temperature and the oxide thengrows on the substrate. In the embodiment in which the substrate issilicon, the first oxide layer may comprise silicon dioxide (SiO₂). Thethickness of the first oxide layer will typically be about 2000angstroms to about 8000 angstroms. The first oxide layer serves as adielectric insulating layer and provides a means for subsequent definingand etching of the trench. Standard photolithography techniques may beused to pattern the region within the first oxide layer that will definethe trench. A conventional wet etch process may then be used to developthe trench through the first oxide layer and into the substrate. Theresulting trench typically has a depth within the substrate of about 10microns to about 20 microns. The trench will also typically have alength slightly longer than the desired predetermined length of thecomposite beam and a width consistent with the maximum deflection of thebeam, generally about 100 microns.

FIG. 5B illustrates the first microelectronic substrate having a trenchdefined therein after a second microelectronic substrate 206 has beenattached and polished back. The second microelectronic substrate willsubsequently form the first layer of the composite beam and a portion ofthe beam anchor. In a preferred embodiment the second substrate willcomprise silicon. A standard fusion bonding process is used to affix thesecond microelectronic substrate to the first microelectronic substrateat the oxide layer interface. After the second microelectronic substrateis bonded it is polished back to the desired predetermined thickness.The thickness of the second microelectronic substrate will be consistentwith the desired thickness or height of the resulting composite beam.Typically, the second microelectronic substrate will be polished back toa thickness of about 25 microns to about 50 microns.

Referring to FIG. 5C shown is an oxide structure 208 formed on thesecond microelectronic substrate 206. The oxide structure(s) generallyoverlie the area that will comprise the composite beam and a portion ofthe anchors. The oxide structure(s) result from a second oxide layer(not shown in FIG. 5C) being disposed on the second substrate. Thesimilar oxidation process to the one previously used for disposing thefirst oxide layer on the first substrate is typically used to disposethe second oxide layer. In the embodiment in which the secondmicroelectronic substrate is silicon, the second oxide layer maycomprise silicon dioxide (SiO₂). The thickness of the second oxide layerwill typically be about 2000 angstroms to about 8000 angstroms. Standardphotolithography techniques may be used to pattern the requisite oxidestructure. A conventional wet etch process may then be used to developthe oxide structure(s). The resulting oxide structure(s) providedielectric separation, preventing subsequent doping in areas underlyingthe oxide structure(s) and electrical isolation between subsequentconductive regions (i.e., doped regions and metallic regions).

Further processing results in the structure shown in FIG. 5D in which aportion of the second microelectronic substrate has been etched away toexpose one side of the thermal actuator. Standard photolithographytechniques may be used to pattern a side of the overall thermalactuator, including a portion of the anchor structure and the firstsidewall 210 of the first layer of the composite beam into the secondsubstrate. A deep silicon reactive ion dry etch process may then be usedto etch away a portion of the second substrate and expose a sidewall ofthe first layer of the composite beam and a portion of the anchorstructure. A dry etch process is preferred at this stage to create thehigh aspect ratio of the composite beam (about 25-50 microns in depthrelative to an about 5-7 microns width).

Referring to FIG. 5E, shown is the thermal actuator construct afterexposed silicon surfaces have been subjected to a conventional diffusiondoping process. The doping process provides for a continuous conductivepath along the periphery of the composite beam and defines the contactson the anchor. In a typical doping process phosphorus may be used as thedoping impurity, although other materials may be used to create a highlydoped region in the second silicon substrate. The doping process willcreate doped regions in all those area not protected by an oxide. Asshown in FIG. 5E the resulting doped regions may include the firstsidewall 210 of the first layer of the composite beam, the surface ofthe trench 204, the exposed portion of the remainder of the secondmicroelectronic substrate 206 and the regions defining the contacts onthe anchor (for the sake of not confusing the reader, the anchorstructure and, thus, the contacts are not shown in the FIG. 5A-5Gillustrations). The doping of the surface of the trench and exposedportion of the remainder of the second microelectronic substrate isincidental. The doped region will typically have a depth into thesubstrate of about 2000 angstroms to about 8000 angstroms.

FIG. 5F depicts the thermal actuator after an additional etch processhas revealed the definition of the second side of the thermal actuator.After the completion of this etch process all that remains of the secondsubstrate are those structures of the thermal actuator; the first layerand a portion of the anchor structure, that comprise the material of thesecond substrate. FIG. 5F illustrates the definition of the secondsidewall 212 of the first layer after completion of the additional etchprocess. Standard photolithography techniques may be used to patterninto the substrate the remaining side of the overall thermal actuator,including a portion of the anchor structure and the second sidewall ofthe first layer of the composite beam. A conventional reactive ion dryetch process may then be used to etch away the remainder of the secondsubstrate and expose the second sidewall of the first layer of thecomposite beam and the remainder of the anchor structure.

The thermal actuator of the present invention is shown in its completedform in FIG. 5G. A second layer 214, typically a metallic layer isdisposed on the second sidewall 212 of the first layer. The second layermay comprise gold, nickel or another suitable material that has acoefficient of thermal expansion that differs from the coefficient ofthermal expansion of the material chosen to comprise the first layer. Inthe embodiment in which the second layer comprises gold a conventionalevaporation process is used to dispose the layer, typically the layer isabout 2 to about 3 microns thick. In order to properly dispose thesecond layer on the vertical sidewall of the first layer and to assureproper uniform thickness of the second layer the overall thermalactuator construct may be placed at an angle during the evaporationprocess. The evaporation process will result in the second layer beingdisposed on surfaces not requiring such. In order to remove unnecessarysecond layer coverage, a standard photolithography process is used topattern the areas requiring the second layer and a conventional wet etchprocess is used to define the areas requiring second layer coverage. Inmost instances, the etch process will result in second layer coverageextending above the second sidewall 212 and partially covering thesurface of the second oxide layer 208. Additionally, the second layermay remain at the tip of the distal end of the composite beam (as shownin FIG. 1). Alternately, the tip of the distal end of the composite beammay comprise a doped region of the first layer. In the embodiment shownin FIG. 5G the first and second oxide layers 202 and 208 remain on thecompleted thermal actuator. It is also possible and within the inventiveconcepts herein disclosed to remove the oxide layer after the secondlayer has been disposed. The plan and cross-sectional views of FIGS. 1and 2 illustrate an embodiment in which the oxide layers have beenremoved. In most instances leaving the oxide layers intact may bedesirable as it eliminates the need for further processing and mayprevent possible electrical shorting.

Accordingly, the fabrication method of this aspect of the presentinvention provides an efficient and repeatable technique for creating avertical layered MEMS structure having a doped region that provides fora self-contained heating mechanism. The resulting MEMS thermal actuateddevices are capable of generating large displacement forces across anin-plane, generally linear direction. These devices benefit from thecapability of being able to operate at significantly lower temperatureand power while consuming less surface area on the substrate.

Many modifications and other embodiments of the invention will come tomind to one skilled in the art to which this invention pertains havingthe benefit of the teachings presented in the foregoing descriptions andthe associated drawings. Therefore, it is to be understood that theinvention is not to be limited to the specific embodiments disclosed andthat modifications and other embodiments are intended to be includedwithin the scope of the appended claims. Although specific terms areemployed herein, they are used in a generic and descriptive sense onlyand not for purposes of limitation.

That which is claimed:
 1. A microelectromechanical (MEMS) actuatorstructure comprising: a microelectronic substrate having a firstsurface; an anchor structure affixed to the first surface of saidmicroelectronic substrate; and a thermally actuated composite beamhaving a first layer with a first thermal coefficient of expansion and asecond layer with a second thermal coefficient of expansion that islower than the first thermal coefficient of expansion and having aproximal end that is coupled to said anchor structure and a distal endthat is cantilevered over the first surface of said microelectronicsubstrate, wherein the distal end of said composite beam movessubstantially parallel to the first surface of said microelectronicsubstrate in a direction that is towards the second layer in response toheat applied thereto.
 2. The MEMS actuator of claim 1, wherein saidcomposite beam further comprises an electrically conductive path throughsaid composite beam, such that said composite beam is thermally actuatedin response to electrical current flowing therethrough.
 3. The MEMSactuator of claim 2, wherein said anchor further comprises at least twocontacts in operable connection with the electrically conductive path.4. The MEMS actuator of claim 1, wherein the first and second layers areboth disposed in a generally vertical relationship with respect to thefirst surface of said microelectronic substrate.
 5. The MEMS actuator ofclaim 1, wherein one of the first and second layers comprises asemiconductor material.
 6. The MEMS actuator of claim 5, wherein thefirst layer including a semiconductor material further comprises acontrollably doped region so as to impart self-heating capabilities tosaid composite beam.
 7. The MEMS actuator of claim 5, wherein the firstlayer includes a semiconductor material having a controllably dopedregion so as to impart self-heating capabilities to said composite beam.8. The MEMS actuator of claim 1, wherein the composite beam furtherincludes a semiconductor material layer having a controllably dopedregion.
 9. The MEMS actuator of claim 1, wherein the composite beamfurther includes a metallic material layer.
 10. The MEMS actuator ofclaim 1, wherein one of the first and second layers comprises a metallicmaterial layer.
 11. The MEMS actuator of claim 1, wherein the firstlayer comprises a semiconductor material and the second layer comprisesa metallic material, wherein the second layer has a greater thermalcoefficient of expansion than the first layer, such that said compositebeam deflects toward the first layer at the distal end in response toselective thermal actuation.
 12. The MEMS actuator of claim 1, whereinsaid microelectronic substrate further defines a trench generallyunderlying said composite beam.
 13. The MEMS actuator of claim 1,wherein said anchor further comprises a first anchor portion and asecond anchor portion which are physically separated by an air gap. 14.The MEMS actuator of claim 13, wherein said first anchor portion furthercomprises a first contact and said second anchor portion furthercomprises a second contact, such that an electrically conductive path isinitiated at the first contact, travels through said composite beam andreturns via the second contact.
 15. The MEMS actuator of claim 14,wherein the first and second contacts further define controllably dopedregions within a semiconductor material.
 16. The MEMS acuator of claim1, wherein said acuator is thermally actuated by a heated methodselected from the group consisting internal heating, external heatingand ambient temperature variations.
 17. A microelectromechanical (MEMS)actuator structure comprising: a microelectronic substrate having afirst surface; at least two anchor structures affixed to the firstsurface of said microelectronic substrate; and at least two thermallyactuated composite beams, each composite beam having a first layer witha first thermal coefficient of expansion and a second layer with asecond thermal coefficient of expansion that is lower than the firstthermal coefficient of expansion and each composite beam having aproximal end that is coupled to at least one of said anchor structuresand a distal end overlying the first surface of said microelectronicsubstrate, wherein the distal ends of said at least two thermallyactuated composite beams move substantially parallel to the firstsurface of said microelectronic substrate in a same direction that istoward the second layers of the at least two thermally actuatedcomposite beams in response to heat applied thereto.
 18. The MEMSactuator structure of claim 17, wherein said at least two compositebeams further comprises a first and a second composite beam disposedupon said microelectronic substrate such that the distal ends of saidfirst and second composite beams face each other, and wherein the firstand second composite beams move in unison in response to thermalactuation thereof.
 19. The MEMS actuator structure of claim 18, whereinsaid first and second composite beams are operably connected at thedistal ends by an interconnecting element, such that upon thermalactuation of the composite beams the interconnecting element will movein a generally linear direction.
 20. The MEMS actuator device of claim19, wherein said interconnecting element has a wishbone-likeconfiguration so as to provide flexing capabilities and impart a greaterlinear displacement distance.
 21. The MEMS actuator device of claim 19,wherein said interconnecting element is controllably doped so as topermit an electrical path between said first and second composite beams.22. The MEMS actuator device of claim 19, wherein the first and secondcomposite beams are disposed proximate to a flexible beam structurecomprising at least two anchors affixed to said microelectronicsubstrate and a platform operably connected between the at least twoanchors by flexible beams, and wherein the platform is disposedproximate the distal ends of the first and second composite beams, suchthat the platform will be deflected linearly in response to thermalactuation of the first and second composite beams.
 23. The MEMS actuatorof claim 17, wherein said at least two composite beams further compriseelectrically conductive paths through each composite beam, such thatsaid composite beams are thermally actuated in response to electricalcurrent flowing therethrough.
 24. The MEMS actuator of claim 23, whereinsaid at least two anchors further comprises at least two contacts inoperable connection with the electrically conductive paths.
 25. The MEMSactuator of claim 17, wherein said at least two composite beams furthercomprise, individually, at least two layers which have differing thermalcoefficients of expansion so as to respond differently to thermalactuation.
 26. The MEMS actuator of claim 25, wherein the at least twolayers are both disposed in a generally vertical relationship withrespect to the first surface of said microelectronic substrate.
 27. TheMEMS actuator of claim 25, wherein the at least two layers furthercomprises a first layer including a semiconductor material.
 28. The MEMSactuator of claim 27, wherein the first layer including a semiconductormaterial has a controllably doped region so as to impart self-heatingcapabilities to said composite beam.
 29. The MEMS actuator of claim 25,wherein one of the at least two layers comprises a metallic material.30. The MEMS actuator of claim 25, wherein the at least two layersfurther comprise a first layer including a semiconductor material and asecond layer including a metallic material, wherein the second layer hasa greater thermal coefficient of expansion than the first layer, suchthat said composite beams deflect toward the first layer at the distalend in response to selective thermal actuation.
 31. The MEMS actuator ofclaim 30, wherein the first layer including a semiconductor material hasa controllably doped region so as to impart self-heating capabilities tosaid composite beams.
 32. The MEMS actuator of claim 31, wherein thecontrollably doped region of said first layer is operably in contact atthe distal end of said composite beams with said second layer, such thata conductive path of varying resistance is provided by said compositebeams.
 33. The MEMS actuator of claim 17, wherein said microelectronicsubstrate further defines at least two trenches generally underlying theat least two composite beams.
 34. A system for MEMS thermal actuation inan in-plane direction, the system comprising: a voltage source; and aMEMS actuator device operably connected to said voltage source, saidMEMS actuator device comprising: a microelectronic substrate having afirst surface; at least one anchor structure affixed to the firstsurface, said at least one anchor structure having at least twoelectrical contacts; and a thermally actuated composite beam having afirst layer with a first thermal coefficient of expansion and a secondlayer with a second thermal coefficient of expansion that is lower thanthe first thermal coefficient of expansion and having a proximal endthat is coupled to said at least one anchor structure and a distal endoverlying the first surface, said thermally actuated composite beamincluding an electrically conductive path electrically coupled to saidvoltage source, wherein the thermally actuated composite beam is heatedby a current from said voltage source to the electrically conductivepath via the electrical contacts, wherein the distal end movessubstantially parallel to the first surface of said microelectronicsubstrate in a direction that is towards the second layer in response tothe heat applied thereto.